If I spill a small amount of water (or other similar liquid) on the worktop and leave it, it will eventually evaporate into vapour, as I'm sure everyone has seen. But the room is not at 100 degrees celsius (boiling point of water) so how can it evaporate?

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• Right, so as I asked the previous answerer, when the molecules evaporate are they actually boiling or not?If so, is 100 degrees classed as the temperature at which all the molecules will have boiled? If not, what does the boiling point signify that is different from normal evaporation into vapour? Thanks.

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  by ChrisDG on December 10th, 2006

• I'm not sure our research techniques are refined enough to know what happens to individual molecules, and I'm not a scientist so I can't easily lay my hands on the average speed in feet per second of a water molecule at 100 degrees though that information does exist somewhere. What the boiling point is, is a temperature at which a substance's molecules are moving so fast that there are few connections holding them together, so much so that they spontaneously evaporate and become a gas even inside the liquid itself. Since evaporation only takes place at the surface, and since boiling increases the surface area, it increases the speedof evaporation. The boiling TEMPERATURE is significant because it is where the van der Waals forces holding water together as a liquid are broken. If you have a frozen pot of water and put it on a stove, the temperature will rise steadily except for a long plateau at zero degrees while it melts and another long plateau at 100 degrees.

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  by Roger Kovaciny on December 10th, 2006

• Van der Waals forces, mentioned above, are similar to but weaker than the forces that bind hydrogen and oxygen together to form water. The hydrogens sticking out at the end of a water molecule are strongly attracted to the oxygen atoms in other water molecules. These are stronger in water than in any other substance, and explain why water will soak up more heat both at its melting point ("heat of crystallization") and at its boiling point ("heat of vaporization") than any other substance. What else is significant about the boiling point is that you can't really increase the temperature of water beyond it except by putting it in a pressure cooker--you just increase the speed at which it evaporates. At a high enough temperature, it boils so fast it practically explodes.

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  by Roger Kovaciny on December 10th, 2006

• I knew about Van der Waals forces and hydrogen bonds from A level chemistry, just my question suddenly occurred to me the other day and I wasn't sure about the answer. Good explanation, thanks.

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  by ChrisDG on December 11th, 2006

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• OK, so you mean that they don't actually boil, they just "move" into the air with the kinetic energy? And my kitchen is not a vacuum, and is at 1 atmosphere but the water still evaporates. How if that temperature is not reached?

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  by ChrisDG on December 9th, 2006

• Very True: This totally agrees with what I was taught in introductary college chemistry.

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  by BioNowCommMajor on December 10th, 2006

• The statement that surface molecules have more kinetic energy that molecules below the surface is not true. Temperature is a measure of the average kinetic energy of the molecules in a sample. Surface molecules will have the same temperature as interior molecules once an equilibrium between evaporation and re-condensation has been reached. During active evaporation, the surface might, if anything, be cooler than the interior, because it is the "hotter", more energetic molecules that escape from the liquid. Molecules at the surface and in the interior are constantly changing places. The reason that water can evaporate at temperatures below the boiling point has already been given in earlier answers, but not in quite the following terms. At a given temperature, there is a minimum energy needed for a water molecule to escape from the liquid. The higher the temperature, the greater fraction of molecules have this minimum energy.

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  by omnicurious on December 12th, 2006

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